The present invention relates generally to storage battery systems having high energy storage per unit weight and, in particular, to the formation of positive lithium electrodes for use in such systems.
A battery stores energy by employing a reducing agent such as lead as an anode or negative electrode and an oxidizing agent such as lead dioxide as the cathode or positive electrode. An electrolytic solution such as sulfuric acid in water is used between the electrodes. When energy is withdrawn, the reducing agent (lead) gives up electrons which flow through an external circuit and are received by the oxidizing agent (lead dioxide) at the positive electrode. Ions, such as hydrogen ions and sulfate ions must flow through the electrolytic solution between the electrodes to complete the circuit. Some type of chemical compound, such as lead sulfate, is produced as a result of the combination of these processes. The product or discharge compounds are stored usually in a porous structure of one or both of the electrodes.
As the above processes take place there is a net overall decrease in the chemical energy within the cell. Some of this energy is recovered to do useful work and some to drive the processes occurring within the battery. If the electrodes contact one another a short circuit causes the battery to discharge and rapidly run down. Separators or membranes therefore are placed between the electrodes, although, of course, the membranes must allow ion transport. Also, chemical shorting may result when the oxidizing agent is dissolved in the electrolytic solution and this problem is resolved by using oxidizing agents which are insoluble or nearly insoluble in the solution.
For a successful storage battery system to work, the above-described processes must occur in reverse. By using an external power supply, the flow of electrons should be reversible to the extent that the reducing agent and oxidizing agent are reformed at the negative and positive electrodes. In some systems, metal reducing agents, such as zinc or lithium tend to grow as so-called dendrites across the gap between the electrodes. When dendrites bridge the gap between the electrodes, a short occurs unless suitable membranes are deployed.
Energy-storing, reversible systems, such as the batteries used for vehicle propulsion or for powering small boats, portable tools, appliances, space satellites, etc., are designed to provide maximum energy storage per unit weight of the battery. In other words, these batteries must have a high energy and power density. Other design considerations involve the life of the battery, its cost and, of course, the operating efficiency. Usually, there is some sort of trade-off between these desirable characteristics.
To provide batteries capable of storing more energy per unit weight, it is desirable to utilize negative electrodes formed of the more powerful reducing agents which have relatively lower equivalent weights than, for example, conventional lead or cadmium. Also, the positive electrodes should use oxidizing agents which again are more powerful and of lower equivalent weights than lead dioxide or nickle oxide. Lithium, as is knwon, is one of the strongest reducing agents and it has an equivalent weight of only 7. It also can become relatively economical since its availability is about the same as lead and its cost, therefore, can be expected to drop with increased demand.
Lithium, therefore, appears to be a highly desirable reducing agent and this desirability has been recognized rather extensively in the available literature. For example, its use is discussed at some length in the following references:
Jasinski, Raymond, High Energy Batteries, Plenum Press, New York, 1967. PA0 Jasinski, Raymond, "Electrochemical Power Sources in Nonaqueous Solvents", Electrochem. Technology, 6, 28-35 (1968) PA0 Tiedemann, W. H. and D. N. Bennion. "Chemical and Electrochemical Behavior of Lithium Electrodes in Dimethysulfite Electrolyte Solutions," J. Electrochem. Soc., 120, pp. 1624, 1973. PA0 Ubbelohde, A. R., "Overpotential Effects in the Formation of Graphite Nitrates," Carbon, 7, 5234530 (1968). PA0 Croft, R. C., "Lamellar Compounds of Graphite," Quarterly Reviews (London), 14, 1-45 (1960). PA0 Braeuer, Klaus, H. M., "Reserve Type Organic Electrolyte Batteries", 4th Inter-Society Energy Conversion Engr. Conf., Washington, D. C., Sept. 1969, pp 525-527 Watanabe, Nobuatsu and Masataro Fukuda, "Primary Cell for Electric Batteries", U.S. Pat. No. 3,536,532, Oct. 27, 1970 Fukuda, Masataro and Taskashi Iijima, "The Lithium/Poly-Carbon Monofluoride Battery," Abstract 41, Fall Meeting The Electrochemical Society, Oct. 1971
However for a variety of reasons no practical storage battery yet has been demonstrated using lithium negative or positive electrodes. The principle reason apparently has been the inability to develop suitable positive electrodes. In this regard, it is known that lithium reacts spontaneously with water and protons. Consequently, electrolytic solutions for lithium systems must be nonaqueous, aprotic solutions and one of the difficulties has been the problem of providing a positive electrode capable of operating reversibly in the nonaqueous solvents. In particular, the positive electrodes should be sufficiently insoluble to prevent chemical shorting. The literature, as already noted, apparently does not describe significantly successful results. Most of the effort has gone into using chlorides, fluorides, or sulfides of copper, silver or nickle. Oxides of molybdenum also have been tried but, in general, these compounds either dissolve in the electrolytic solution or simply are inactive.
Graphite intercalation positive electrode compounds also have been studied. For example, work in this area is described in:
Similar work also is described in U.S. Pat. No. 3,844,837 "Non-Aqueous Battery" issued Oct. 29, 1974 to Bennion et al. This patent, in particular, describes the electrochemical formation of intercalated compounds of graphite with perchlorate ions from non-aqueous aprotic solutions. Generally, the electrochemical reaction at the positive electrode appears to be: EQU nC+C10 .sub.4.sup.- .fwdarw.C.sub.n C10.sub.4 30 e.sup.-.
However, it also has been found that there is a rather significant limitation in that the above reaction proceeds well only when n is greater than 90. For an n less than 90, solvent decomposition apparently occurs at the same time as intercalation is occuring. Such a large value of n coupled with the solvent decomposition seriously detracts from the practical potential of these batteries particularly when high energy density is a design factor.
It also is known that fluorine forms a special class of intercalation compounds with graphite and this material has been used successfully for making primary but not storage batteries. When used for primary batteries, the intercalation compounds has been formed by gas phase fluorine reacting with small particles of graphite. There has been no indication in the literature that such compounds can be formed electrochemically to provide reversible positive electrodes for storage battery systems. The previous work in this field is reported in:
The electrochemical formation of carbon fluoride compounds (C.sub.n F).sub.x clearly would open the way for development of storage batteries using this compound as the active material in the positive electrodes. Such a development would be useful since, if n is small, (C.sub.n F).sub.x has a very low equivalent weight and it is a strong oxidizing agent. Further, it is insoluble and for n close to, but greater than, one it has relatively good conductivity.
The present invention therefore has as its primary object the electrochemical formation of an energy-storing reversible electrode for use in a battery system. In general, this object is accomplished by applying the charging current through a non-aqueous lithium salt solution to an electrode compounded from graphite and lithium fluoride. It appears that such a process is capable of electrochemically forming (C.sub.n F).sub.x compound where n goes from a rather large number to some value between 1 and 2.